1. Field of the Invention
The present invention relates to the art of filtration and, particularly, to improving the structural integrity of filter elements comprising membranes supported from a base plate. More specifically, this invention is directed to filter elements for use in press-type filters and, especially, to membrane filter plates having a base plate member which reliably engages and supports a membrane, particularly a flexible elastomeric membrane, about the periphery of an opening provided in the plate. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in multiple plate filters and, especially, in filter presses. Filter elements comprising a base plate which supports a membrane are known in the art. For a general discussion of membrane filter plates, reference may be had to U.S. Pat. No. 4,608,164. Such filter elements have a base plate provided with at least one opening, which is coaxial with openings in the other plates of the filter assembly, for defining a fluid feed channel. The membrane, which is typically associated with a filter cloth, engages the base plate about the periphery of the opening thereon the engagement being effected by means of a circumferential edge bead which is held in a groove provided about the plate opening. Retention of the membrane bead in the edge groove of the base plate has customarily been accomplished in the prior art by the use of a clamping ring.
The disengagement of the membrane edge bead from its receiving groove in the filter plate is a problem of long standing in the art. Such disengagement may occur because, during a pressing process, compressed air or some other suitable pressurized fluid will be introduced between the plate and the membrane and will force the resilient membrane away from the plate. Membrane disengagement about the periphery of a filter plate opening which in part defines the feed to a filter assembly has been a particular problem. In the prior art, this problem was sometimes addressed by affixing the membrane to the filter plate in the area of the edge bead through the use of an adhesive or by vulcanizing. These techniques, however, are not reliable because substantial shear forces may be generated due to membrane expansion during a pressing process and, in the region about the periphery of a feed opening, as opposed to the area about the outer edge of the filter plate, there is no structure to support the press membrane and resist such forces.
German Patent 23 24 876 discloses a filter element wherein the filter plate is provided with a circumferential groove which receives an edge bead provided on the membrane. The edge bead is retained in the circumferential groove by means of a slotted snap ring. The snap ring is, in turn, retained in place by means of a closed support ring. The technique for fastening a membrane to a filter base plate as disclosed in German Patent 23 24 876 has the disadvantage that it does not permit the use of very high press pressures. At attempt has been made to overcome this disadvantage by utilizing, instead of the support ring, a snap ring fastening comprising flanges which are bolted to "chamber" plates which are disposed between filter elements. Such flanges contact the snap ring so as to prevent axial movement thereof. The flanges also serve to position and support the filter element and to clamp in place a filter cloth which extends over the press membrane. The technique of using such flanges, however, has the disadvantage that it has very limited utility in the handling of aggressive media. Further, while a pressure load of up to three bar on the membrane is possible, there is an inherent danger that the membrane will be pulled from its anchoring in the presence of higher pressures.
German Patent Application 40 32 937 describes a membrane filter plate for plate filter presses wherein anchoring of the membrane in the area of an opening of a filter plate is achieved by forming the filter plate opening in a conical shape, providing the groove for receiving the edge bead of the membrane in the conical surface area and retaining the bead in the groove by means of a conically shaped clamping ring. In this technique, the outer surface of the clamping ring adjacent to the edge bead of the membrane is inclined away from the mid plane of the filter plate at an angle which is determined by the angle of the wall of the opening in the filter plate. When pressure is applied to the membrane to cause expansion thereof, the membrane will be pushed away from the filter plate in the axial direction, i.e., the membrane will be urged against the conical bearing surface of the clamping ring. In order to prevent the membrane from disengaging from the receiving groove of the filter plate, the clamping ring is secured in place by means of bolts which cooperate with a holding cross-piece portion of the filter plate. Accordingly, axial motion of the clamping ring is prevented and the edge bead of the membrane will be securely captured in the receiving groove. In order for this technique of ensuring against disengagement of the membrane from the filter plate to be successful, it is necessary to employ several mechanical fasteners distributed about the circumference of the clamping ring to retain the clamping ring in position. Since the liquids being treated are frequently highly corrosive, each of these mechanical fasteners must be sealed hermetically with respect to the feed channel of the filter assembly. This requirement for hermetic sealing increases both manufacturing and assembly costs.